The present invention relates to a combined cup for beverages or packing material that consists of an external fibrous material container, made e.g. of cardboard, or impregnated or laminated paper and a thin-walled container of thermoplastic sheeting or film which is drawn or inserted into this cardboard container. Such containers are, for instance, the subject matter of German Pat. No. 1 761 693, which, besides other teachings, proposes to carry out the formation of the rounded cup lip or rim, after the deep drawing process has taken place, by means of which the plastic container is drawn into the paper container, jointly with the paper container and the plastic container. In the course of forming the rounded lip the difficulty arises that the rounded lip, after some time has elapsed, frequently unrolls itself again. A rolled lip that has been rolled up correctly and tightly makes a significant contribution towards strengthening the container. It renders possible a saving in both cardboard weight and cardboard wall thickness. However, this strengthening is cancelled or lost again when the tightly rolled rounded lip unrolls itself again. Moreover, when the rounded lip undergoes any change, the outer dimension of the container frequently changes as well, which may result in difficulties when fitting a lid, an operation which only takes place at the filling station, after the container has been transported through entirely different climatic conditions and through various degrees of atmospheric humidity.
It is the object of the present invention to eliminate this shortcoming. In this connection one starts from the finding that the unrolling or rolling back of these rounded lips is to be ascribed to two causes as detailed below.
1. The atmospheric humidity penetrates through the front side of the cardboard at the rounded lip. In most cases involving modern technology, the cardboard is, at least on the inside, provided with a coating that is both airproof and impervious to liquids, e.g. a coating of polyethylene and the outside is also protected either by the application of a suitable surface pressure or by a surface varnishing or also merely by an application of chalk which results in a smooth surface being obtained which prevents the atmospheric humidity from so readily penetrating into the cardboard. However, the cutting edge at the topmost portion of the blank from which the cup was produced, the portion that is utilized for the formation of the rounded lip and, even when the rounded lip is rolled up tightly, this cutting edge nevertheless remains exposed to the atmosphere without any protection. Added to this is the circumstance that, due to the cut, the fibres of the paper have been cut through as well and, as a consequence hereof, they have acquired certain hygroscopic properties, thus directly attracting the humid outside atmosphere. If moisture now penetrates into the cardboard at the location of the rounded lip, the fibres swell up, the lengths within the cardboard increase and the rounded lip thus assumes a position in which it is possible for the upper edge to be larger. Consequently, it unrolls itself again.
2. The second cause is provided by the resilient properties of the plastic sheeting. Already during the deep drawing process the plastic sheeting is subjected to certain thermal processes as well as to tensile and deformative stresses which, in turn, lead to internal stresses arising in the sheeting. As soon as the sheeting is exposed to a more substantial temperature fluctuation, for instance, while being transported or when being filled with hot contents, or on other occasions, these stresses are set free and the sheeting attempts to shrink. This shrinkage does then under certain circumstances likewise lead to the rounded lip unrolling itself again if no specific provision is made that is aimed at causing the shrinkage forces to result in a strengthening of the flanging rather than in an increase in size.
As it is, the invention teaches that the internal inset of plastic material is not to be cut off exactly flush with the upper rim of the paper cup, but that the inner cup is left longer by approximately 2 mm; at least, however, by double the paper thickness. To begin with, this does contradict what is being normally aimed at, since ordinarily it is assumed that such a thin, projecting flap would be more likely to tend to have an impeding effect. Thus all kinds of special mechanical measures have been taken to ensure that the sheeting is actually cut off as flush as possible with the cup rim. If, however, the sheeting is allowed to project somewhat over the cup rim, then it is possible to eliminate both causes of the rounded lip unrolling itself by means of this measure and, to be more precise, in that the flap wraps itself around the front edge of the cardboard cup when warmth or heat is applied for the first time due to shrinkage forces and, in this manner, protects said front edge against the penetration of humid air. However, once the rounded lip has then been formed, the shrinkage forces, when heat is applied for a second time, act upon the sheeting portions inside the rounded lip and these, in turn, act in part antagonistically in relation to each other. That portion which is located above the rounded lip will, due to shrinkage, be drawn towards the center axis of the cup and is, on account of this, going to attempt to unroll the rounded lip again, while the lower portion, which is likewise drawn towards the centre axis due to shrinkage, will be tightening the rounded lip more firmly again. The projecting flap will now confer a predominance to the tightening portion of the rounded lip, so that the tightening forces will gain the upper hand and will consequently strengthen the rounded lip. This process will be explained with the aid of the following figures.